Callout loop not allowed error

I have an APEX REST POST method that is being called from an external system ‘A’.

Processing this POST I need to callout to another external system ‘B’ and based on its response continue with my application logic, eventually returning the result to system ‘A’.

The problem is that I’m getting an Error: “System.CalloutException: Callout loop not allowed”

is there any way to get around this ?

Answer

No, there is not an immediate workaround to this 🙁

The mechanism by which they measure that ‘callout depth’ is by transmitting an HTTP header on requests originating from Apex, Sfdc-Stack-Depth: 1 – if you try and trick the platform by explicitly setting this header to '' etc, you’ll find the value is very sticky!

Unfortunately for us, there is a good business reason behind this “feature” on Salesforce’ part. It is by design, to prevents callouts from blocking threads between multiple orgs / instances.
sequence diagram

(hopefully this sequence diagram illustrates the intention; as an HTTP callout could take a while, the number of loopback connections is restricted to 1, otherwise imagine the DoS implications)

You might be able to overcome this issue if your B callout were read-only and its contents were updated infrequently.

You could schedule a batch process to regularly perform the callout and avail its response in a Document (or such). Then read that Document when responding to A, decoupling the two. But I admit that may be unlikely in your case…

Attribution
Source : Link , Question Author : Prafulla Patil , Answer Author : Matt and Neil

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